It is well known that when either a smoker or a smoking machine smokes a conventional filter-tipped cigarette, the amount of "tar" in each puff increases in each successive puff. The "tar" yield of the last few puffs is usually several times higher than that from the first few puffs. Consequently the "tar" in the last few puffs may be perceived by a smoker as "too strong", and a smoker, therefore, may consider a cigarette as "smoother", if the "tar" delivery profile can be made more even.
There have been many attempts in the past to achieve an "even" puff cigarette by modifying the filter. Mullor, U.S. Pat. No. 2,763,267 and Clayton et al, U.S. Pat. No. 3,809,097 proposed to use a plurality of discs to be placed inside a cigarette filter rod and placed transverse to the smoke flow. These designs, however, can only improve filtration efficiency, but without any ability of evening the cigarette "tar" profile.
Patterson, U.S. Pat. No. 3,648,712 proposed to use a low gas permeability disc placed transversely inside an ordinary cellulose acetate filter rod. The disc is of a diameter which is 5 to 25 percent less than the diameter of the filter rod and this filter, therefore, can only achieve improved filtration efficiency, with no ability to even the "tar" delivery profile.
B.A.T. GB Pat. No. 1,339,238 proposed placing a disc of a blocking plug between a tobacco rod and an ordinary cellulose acetate filter rod. This disc has an orifice for smoke to flow through. The disadvantage of this design is that the pressure drop of the cigarette increases to an unacceptable level as the orifice of the plug becomes clogged up at the later puffs.
Brackmann et al,. U.S. Pat. No. 3,882,877 proposed placing a disc of microfine fibres in between two filter rod segments. This filter is designed to increase filtration efficiency. However, it could not cause the "tar" delivery to be more even, furthermore the pressure drop may increase to an unacceptable level at the later puffs.
Browne et al., U.S. Pat. Nos. 4,460,000 and 4,469,112 proposed using a compound filter that contains a perforated barrier disc through which the whole smoke passes to a cellulose acetate filter rod segment. As the smoking process proceeds, more and more "tar" builds up on the cellulose acetate fibres around the exit of the perforations. The disadvantage of this design is that the build-up of "tar" near the perforations will greatly increase the flow resistance of the cigarette filter to an unacceptable level.
G.B. Pat. Nos. 2,102,271 A, 2,103,065 A, and 2,105,566 A of Filtrona (U.K.), proposed to employ two flow paths, i.e. a low pressure drop path which has low filtration efficiency and a high pressure drop path which has high filtration efficiency, for the smoke to flow in a filter rod. During the initial few puffs of a cigarette, it is intended that the smoke flow through the low pressure drop path. The smoke then switches to flow through the high pressure drop path due to the presence of a blocking mechanism on the low pressure drop path. The disadvantage of these filters is that they are very complicated and difficult to manufacture.
U.S. Pat. No. 4,747,418 uses a filter element in the form of a membrane having a large number of tiny openings in combination with openings downstream of the membrane through which air may be drawn or conventional filter substance in an attempt to produce a more uniform tar delivery. Again a complicated and expensive structure is required.